38 MAGNIFICA TION AND DRA WING. 



should be examined and if it is found satisfactory, /. e., if the lines are 

 black, a cover-glass on which is a drop of warm balsam may be put 

 over the lines to protect them. 



§ 102. Determination of Magnification. — This is most readily ac- 

 complished by the use of some form of camera lucida (§§ 121, 122), 

 that of Wallaston being most convenient as it may be used for all pow- 

 ers, and the determination of the standard distance of 250 millimeters 

 at which to measure the image is very readily determined (PI. Ill, 

 Fig. 27, § 104). 



Employ the 18 mm. (^ in.) objective and a 50 mm. (2 in., A or No. 

 1) ocular and stage micrometer as object. For this power the y^th mm. 

 spaces of the micrometer should be used as object. Focus sharply, and 

 make the body of the microscope horizontal, by bending the flexible 

 pillar, being careful not to bring any strain upon the fine adjustment 

 (§71, Fig. 10). 



Put a Wallaston's camera lucida (§ 121) in position, and turn the 

 ocular around if necessary so that the broad flat surface may face di- 

 rectly upward as shown in Fig. 27. Elevate the microscope by putting 

 a block under the base, so that the perpendicular distance from the up- 

 per surface of the camera lucida to the table is 250 mm. (§ 104). Place 

 some white paper on the work-table beneath the camera lucida. 



Close one eye, and hold the head so that the other may be very close 

 to the camera lucida. L,ook directly down. The image will appear to 

 be on the table. It may be necessary to readjust the focus after the 

 camera lucida is in position. If there is difficulty in seeing dividers and 

 image consult § 121. Measure the image with dividers and obtain the 

 power exactly as above (§§ 98, 99). 



Thus : Suppose two of the yVth mm - spaces were taken as object, and 

 the image is measured by the dividers, and the spread of the dividers 

 is found on the steel rule to be 9A millimeters. If now the object is T^ths 

 of a millimeter and the magnified image is 9-f millimeters the magnifi- 

 cation (which is the ratio between size of object and image) must be 

 9! -r- t 2 q = 47. That is, the magnification is 47 diameters or 47 times 

 linear. If the fractional numbers in the above example trouble the stu- 

 dent, both may be reduced to the same denomination, thus : If the size 

 of the image is found to be gf mm. this number may be reduced to tenths 

 mm. so it will be of the same denomination as the object. In 9 mm. 

 there are 90 tenths, and in -§ there are 4 tenths, then the whole length 

 of the image is 90 -f- 4 = 94 tenths of a millimeter. The object is 2 

 tenths of a millimeter, then there must have been a magnification of 

 94 -r- 2 = 47 diameters in order to produce an image 94 tenths of a mil- 

 limeter long. 



Put the 25 mm. (1 in. C or No. 4) ocular in place of one of 50 mm. 



